Add ServiceState type

[atc0005]

Currently used by the atc0005/check-cert project:

// ServiceState represents the status label and exit code for a service check.
type ServiceState struct {

    // Label maps directly to one of the supported Nagios state labels.
    Label string

    // ExitCode is the exit or exit status code associated with a Nagios
    // service check.
    ExitCode int
}